Method of separating a trivalent actinide from a trivalent lanthanide and/or yttrium in aqueous solution

ABSTRACT

The invention relates to a method of separating trivalent actinides from at least one trivalent lanthanide and/or yttrium, whereby the trivalent actinides are extracted from an aqueous solution containing a H +  concentration of 0.01 to 2 moles/liter by using as extractant an (aryl)dithiophosphinic acid of the Formula R 1 R 2 PS (SH), in which R 1  is phenyl or naphthyl, R 2  is phenyl or naphthyl, or R 1  and R 2  are each phenyl or naphthyl substituted by methyl, ethyl, propyl, isopropyl, cyano, nitro, or halogen, with addition of an extraction synergist selected from the group consisting of trioctylphosphate, tris-(2-propylpentyl)-phosphate, and tris-(2-ethylhexyl)-phosphate. The use of the synergist in the extraction allows for a more selective extraction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to Ser. No. 08/931,894 filed Sep. 17, 1997now U.S. Pat. No. 5,966,584. This application is the U.S. National Phaseof PCT/DE 98/02815 filed Sep. 15, 1998.

FIELD OF THE INVENTION

The invention relates to a method of separating trivalent actinides fromat least one trivalent lanthanide using an extraction-medium containingan extractant and an extraction synergist.

BACKGROUND OF THE INVENTION

The separation of trivalent actinides like americium or curium, fromtrivalent lanthanides has long been a problem. The reason for thedifficulties in the separation of these elements is the very closechemical properties of the trivalent ions of the lanthanides andactinides. Especially the very similar ionic radii contribute to similarproperties of these element groups. Thus numerous investigations havehitherto been undertaken to separate the trivalent ions of the actinidesand lanthanides with the highest possible selectivity and increasedefficiency.

It is indeed known that extraction agents with mild donor groups likenitrogen or sulphur as complex formers contain structural componentswhich in liquid-liquid extraction have a certain selectivity withrespect to trivalent actinides although up to now such separations havebeen carried out only at relatively high pH values which tend toprecipitate formation of the trivalent actinides or require 10 molarLiCl solutions.

In the publication K. L. Nash, Solvent Extr. Ion Exch. 11 (4), 729-768(1993) the Talspeak process is described and enables selectiveextraction of lanthanides with the aid of complexing agents whichmaintain the trivalent actinides in solution. However, this separatingprocess is also carried out at a relatively high pH value of 3-4 andrequires the addition of further salts.

A process developed by the Applicant for the extraction of trivalentactinides from aqueous solutions which contain trivalent actinides andtrivalent lanthanides, enables a separation at high acid concentrationsof 0.01 to 2 mol/liter HNO₃. Following this process, such an aqueousacid solution which contains a mixture of trivalent lanthanides andactinides is extracted by means of an organic solvent. With thisprocess, because of the low pH value or the high acid concentration, aprecipitate formation of the trivalent actinides is hindered and goodseparation results are obtainable. To evaluate the separation results,characteristic values such as the distribution coefficient D and theseparation factor α are considered

D_(An(III))=[An(III)_(org)]/[An(III)_(w)]  (1)

In formula 1:

D_(An(III))=distribution coefficient for a trivalent actinide(dimensionless)

[An(III)_(org)]=concentration of the trivalent actinide in the organicphase (mole/liter)

[An(III)_(w)]=concentration of the trivalent actinide in the aqueousphase

D_(Ln((III))=[Ln(III)_(org)]/[Ln(III)_(w)]  (2)

In formula 2:

D_(Ln(III))=Distribution coefficient for a trivalent lanthanide(dimensionless)

[Ln(III)_(org)]=concentration of the trivalent lanthanide in the organicphase (mole/liter)

[Ln(III)_(w)]=concentration of the trivalent lanthanide in the aqueousphase.

α=D_(An(III))/D_(Ln(III))

α=separating factor (dimensionless) depending upon the process, the useof bis(aryl)dithiophosphonic acid in strongly acid medium can produceseparating factors α lying between about 20-50.

OBJECT OF THE INVENTION

It is therefore the object of the invention to provide a method forextraction of trivalent actinides from a solution containing at leastone trivalent actinide and at least one trivalent lanthanide, and/ortrivalent lanthanum and/or trivalent yttrium and, to provide anextraction agent which enables higher separating factors α to beobtained. It should therefore be possible to carry out the method atlower pH values or higher acid concentrations.

SUMMARY OF THE INVENTION

With the method and extraction agent of the invention it is possible atthe same time to achieve a higher separation factor cx while maintainingextraction conditions at lower pH values or higher acid concentrations.

Advantageously further features of the invention are given in thedependent claims.

The invention will be elucidated below by way of example.

An aqueous phase contains trivalent ions of lanthanides (cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pr), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), luthenium (Lu), as wellas lanthanum (La) and yttrium (Y) or at least one component of thisgroup and ions of trivalent actinides, like americium (Am) and/or curium(Cm). The aqueous phase has a low pH value which can correspond to a H⁺concentration of a strong acid of 0.01 to 2 mole per liter. As acids,HCl, H₂SO₄ and especially HNO₃ can be used since HNO₃ forms readilysoluble salts. The extraction is carried out by means of an organicsolvent which contains as extraction medium bis(aryl)dithiophosphonicacid of the general formula (4)—R₂PS(SH)

where:

R₁=phenyl or naphthyl,

R₂=phenyl or naphthyl, as well as methyl-, ethyl-, propyl-, isopropyl-,cyano-, nitro-, halogenyl- (Cl—, F—, Br—, J—) substituted residues of R₁and R₂ whereby R₁ and R₂ can be substituted with at least one of thecomponents from the group of methyl, ethyl, propyl, isopropyl, cyano,nitro, halogenyl (Cl, F, Br, I).

This group are examples of substituents which increase the acidity ofthe extraction medium bis(aryl)dithiophosphonic acid and whichcontribute to especially good extraction results. What is important isthe increase in the acidity of the extraction medium, the exactpositions of the substituents being of less significance.

According to the invention, the organic phase contains at least onesynergist from the group composed of trioctylphosphate (formula (5)),tris-(2-ethylhexyl)-phosphate (formula (6)) andtris-(2-propylpentyl)-phosphate (formula (7)).

(CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂—O—)₃P=O  (5)

With the addition of the synergists according to the invention there isa significant increase in the separation phase α. The selectivity issignificantly increased in the separation of the actinides from thelanthanides.

The organic solvent is advantageously a solvent containing aromaticcomponents, like benzene, or a pure aromatic solvent.

In an especially advantageous rendering of the invention, at least onecomponent is introduced into the solvent which is selected from thegroup which consists of toluene, xylol, tert.-butylbenzene,bis-(tert.-butyl)benzene or tris-(tert.-butyl)benzene, isopropylbenzene,bis-isopropylbenzene or tris-isopropylbenzene. The introduction of thesesolvents give rise to two additional positive side effects.

The first is a further increase in the selectivity of the An³⁺extraction and the other is an additional improvement in the coefficientdistribution D_(An(III)).

The effectiveness as to the separating factor and the distributioncoefficient properties is in the following sequence:benzene<toluene<tert.-butylbenzene<bisisopropylbenzene,isopropylbenzene, tri-isopropylbenzene, bis-(tert. butyl-benzene),tris-(tert.butyl-benzene).

Below several examples are presented:

In the tables the results of the process of the invention aresummarized:

It shows:

Table 1: The influence of synergists according to the invention upon theextraction.

Table 2: The influence of the solvents on the extraction.

Table 3: The composition of simulated actinide (III)—lanthanide (III)solutions working up for extraction investigations.

Table 4: The selective extraction of Am(III) from the simulatedsolutions of Table 3.

Test Description:

Chemicals and Extraction Medium

The chemicals tributylphosphate (Aldrich, 99%), TOPO(=trioctylphosphineoxide; Merck, p.a. quality), trimethylphosphate(Aldrich, 97%), triphenylphosphate (Aldrich, 98%), trihexylphosphate(Aldrich, 97%), trioctylphosphate (Alfa), tris(2-ethylhexyl)phosphate(Aldrich, 97%) are used as indicated. The solvents toluene, xylene,tert.- butylbenzene and triisoproplybenzene were of p.a. quality. Themixed lanthanide (III) solution of Ce(III), Eu(III), Gd(III), Nd(III),Pr(III), Sm(III), and Y(III), La(III) was prepared by weighing out theirnitrate salts in HNO₃. An isotope tracer Am²⁴¹ (3.7 Mb_(q) in 1 ml of 1M Hcl) and Eu ¹⁵²⁻¹⁵⁴ (3.7 Mb_(q) in 5 ml 0.5 M Hcl obtained from theFirm Blaseg was diluted to 100 ml with distilled H₂O. A 1:1 mixture ofboth solutions (pH 2) was used as the basic solution.

The aromatic dithiophosphonic acid was produced by the method of W. A.Higgins, P. W. Vogel, E. G. Craig, Journal of the American ChemicalSociety 77, 1864-1866 (1955). In good yields, apart from the describedsynthesis of bis(phenyldithiophosponic), the bis(methylphenyl),bis(chlorophenyl) and bis (fluorophenyl)dithiophosphonic acids weremade. The extraction agent was produced by weighing out quantities ofthe dithiophosphonic acid and synergist and dissolving them in anaromatic solvent. The extraction experiments were carried out incentrifuge glasses with Teflon® stoppers. 2 mL of a nitric acid aqueoussolution previously traced with Am-241 and Eu-152 were intensively mixedwith 2 mL of the extraction agent. With centrifugation, the phases wereseparated and then aliquots each of 1 mL were taken from the organic andaqueous phases for analysis. The concentrations of the gamma nuclidesAm-241 and Eu-152 were determined with the aid of gamma spectrometry.The concentrations of the inactive elements La, Ce, Pr, Nd, Sm, Eu, Gdand Y were determined with the aid of ICP-MS.

The following conditions apply to the data given in Tables 1-4:

Table 1

The influence of the synergist on the extraction of Am(III) and Eu(III)with a mixture of 0.5 mol/liter of bis(chlorophenyl)dithiophosphonicacid and 0.25 mol/liter of synergist in toluene from nitric acid.

Table 2

The influence of the solvent on the extraction of Am(III) and Eu(III)with a mixture of 0.5 mol/liter bis (chlorophenyl)dithiophosphonic and0.25 mole/liter and trioctyiphosphinoxide from nitric acid solution.

Table 3

Composition of simulated actinide(III)—lanthanide(III) solutions ofdifferent HNO₃ concentrations from the reclaiming of nuclear fuels bythe PUREX process for extraction tests (IV).

Table 4

The selective extraction of Am(III) from simulated actinide (III)solutions of different HNO₃ concentrations with a mixture of 0.5mole/liter of bis(chlorophenyl)dithiophosphonic acid and 0.25 mol/literof trioctylphosphenyl oxide in tert. butylbenzene.

The method of the invention and the extracting agent enable a highlyeffective separation of the trivalent radioactive reactor from thetrivalent lanthanides which is especially desirable with radioactivereactive wastes.

TABLE 1 Synergist 0.2 mol/liter HNO₃ (0.25 mol/liter) D_(Am) D_(Eu)α_(Am/Eu) Tributylphosphate 2.13 0.068 31.2 Trimethylphosphate 0.00460.00037 12.3 Triphenylphosphate 0.0017 0.00022 8.1 Trihexylphosphate25.54 21.25 1.2 Trioctylphosphate 2.11 0.0088 237.9Tris(2-ethylhexyl)phosphate 1.98 0.00165 1199.8

TABLE 2 HNO₃ Solvent mol/liter D_(Am) D_(Eu) α_(Am/Eu) Toluene 0.5 7.190.306 23.5 1 0.794 0.0732 10.8 2 0.113 0.0329 3.4 3 0.0263 0.0158 1.7xylene 0.5 8.83 0.385 22.9 1 0.927 0.0805 11.5 2 — — — 3 — — —tert.-butylbenzene 0.5 19.18 0.615 31.2 1 1.768 0.0988 17.9 2 0.1930.0393 4.9 3 0.0476 0.0219 2.2 triisopropylbenzene 0.5 55.8 1.23 45.6 14.76 0.159 29.8 2 0.416 0.057 7.3 3 0.106 0.0461 2.3

TABLE 3 HNO₃ in mol/liter 0.5 0.99 2.06 Element in g/L Y 0.2372 0.23450.238 La 0.7446 0.72 0.7409 Ce 1.4595 1.383 1.447 Pr 0.6832 0.59930.6715 Nd 2.4823 2.455 2.4948 Sm 0.4806 0.4685 0.4796 Eu 0.0939 0.08780.0907 Gd 0.0751 0.0765 0.0813 Am-241 Tracer Quantities

TABLE 4 HNO₃ 0.5 mol/l 1.0 mol/l 2.0 mol/l Element D_(M(III)) α_(Am/Ln)D_(M(III)) α_(Am/Ln) D_(M(III)) α_(Am/Ln) Y 0.325 36.7 0.0384 43.6<0.005 — La 0.254 46.9 0.0353 47.4 0.0053 33.8 Ce 0.551 21.6 0.0741 22.60.006 29.8 Pr 0.527 22.6 0.0844 19.9 0.0135 13.3 Nd 0.296 40.3 0.052731.8 0.0099 18.1 Sm 0.373 31.9 0.0627 24.9 0.0099 18 Gd 0.303 39.40.0515 32.5 0.0082 21.8 Eu 0.357 33.4 0.0688 24.4 0.0122 14.8 ¹⁵²Eu0.352 33.8 0.0931 18 0.0377 4.8 ²⁴¹Am 11.926 1.677  0.18

What is claimed is:
 1. A method of separating at least one trivalentactinide element from at least one trivalent lanthanide element and/oryttrium in an aqueous acidic solution containing said elements, saidmethod comprising the steps of: (a) maintaining an H⁺ concentration ofsaid solution in a range of 0.01 moles/liter to 2 moles/liter; and (b)extracting said solution at said H⁺ concentration of 0.01 moles/liter to2 moles/liter with an extractant which consists essentially of: (1) acompound of the Formula (4)

 wherein R₁=phenyl or naphthyl, R₂=phenyl or naphthyl, or R₁ and R₂ areeach phenyl or naphthyl substituted by methyl, ethyl, propyl, isopropyl,cyano, nitro, or halogen, (2) an extraction synergist selected from thegroup consisting of trioctylphosphate, tris-(2-propylpentyl)-phosphate,and tris-(2-ethylhexyl)-phosphate; and (3) an organic solvent or solventmixture.
 2. The method defined in claim 1 wherein the organic solvent isan aromatic compound.
 3. The method defined in claim 2 wherein thearomatic solvent is a compound selected from the group consisting oftoluene, xylene, tert.-butylbenzene, bis-(tert.-butyl)-benzene,tris-(tert.-butyl)-benzene, isopropylbenzene, bis-(isopropyl)-benzeneand tris-(isopropyl)benzene.
 4. The method defined in claim 1 whereinthe trivalent actinide elements americum and curium are separated fromthe trivalent lanthanide elements.
 5. The method defined in claim 1wherein at least one of the trivalent actinide elements and trivalentlanthanide elements is radioactive.
 6. An extraction medium whichconsists essentially of: (1) a compound of the Formula (4)

 wherein R₁=phenyl or naphthyl, R₂=phenyl or naphthyl, or R₁ and R₂ areeach phenyl or naphthyl substituted by methyl, ethyl, propyl, isopropyl,cyano, nitro, or halogen, (2) an extraction synergist selected from thegroup consisting of trioctylphosphate, tris-(2-propylpentyl)-phosphate,and tris-(2-ethylhexyl)-phosphate; and (3) an organic solvent or solventmixture.
 7. The extraction medium defined in claim 6 wherein the organicsolvent is an aromatic compound.
 8. The extraction medium defined inclaim 7 wherein the aromatic solvent is a compound selected from thegroup consisting of toluene, xylene, tert.-butylbenzene,bis-(tert.-butyl)-benzene, tris-(tert.-butyl)-benzene, isopropylbenzene,bis-(isopro-pyl)-benzene and tris-(isopropyl)benzene.